T-Mobile has become the first major cellphone carrier in the United States to begin selling service that allows a single phone to communicate over both cellular networks and Wi-Fi hot spots.

The first phones, which are available to consumers in Seattle on a trial basis, link to T-Mobile’s cellular network outdoors and to Wi-Fi routers in homes, offices and other locations like airports and hotels. The service, introduced Monday, preserves more of customers’ airtime and increases coverage in places where signals are typically weak, like basements and rooms without windows.

To gain access to the service, called T-Mobile HotSpot @Home, customers must buy a phone that works on both networks. T-Mobile is selling two models, each priced at $49.99, to customers who sign up for a two-year rate plan of at least $39.99 a month. Subscribers are charged $19.99 a month in addition to their regular cellular plan fees.

Customers also need a wireless router, which is free after a rebate, that is then connected to any available broadband line for home or office use. The phones connect not just to the wireless router, but also at any of 7,000 Wi-Fi hot spots that T-Mobile operates at Starbucks coffee shops, Hyatt Hotels and other public locations.

T-Mobile has set up a Web site, www.theonlyphoneyouneed.com, for those curious about the service.

Because the service allows customers to make unlimited calls using their broadband connections, it represents a threat to Vonage, SunRocket and other companies that offer phone plans over high-speed Internet connections.

The dual-use phone service could appeal most to younger consumers who do not have a traditional phone line and rely solely on cellular phones and broadband lines.

“For the below-30 age segment, it’s a no-brainer,” said Roger Entner, an industry analyst at Ovum, a consulting firm. “This is also a threat for other wireless carriers because it fixes the problem of poor coverage inside homes.”